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Comparison studies in clinical medicine are typically carried out in one of two settings. In one, an accepted criterion standard exists and is available for comparison; in the other, no independently established criterion standard for comparison exists and a new test or method must be evaluated by comparison to an already established, though possibly inaccurate, technique. For the purposes of this study, we have considered multiwavelength spectrophotometry (eg, CO-Oximetry) as a criterion standard for the determination of hemoglobin oxygen saturation. The statistical methods that we have used to assess agreement between in vivo and in vitro oximetry are based on this assumption. However, for completeness, we have included a “difference versus mean” plot (Fig 1b) as recommended by Bland and Altman.
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The purpose of this study was to compare the clinical performance of in vivo vs in vitro Sv02 determinations in ICU patients using the three most current commercially available in vivo oximetry systems (Oximetrix 3, SAT-2, and HEM0PR02). Evaluation of a monitor requires that both “accuracy” and “reproducibility” are quantified. Previous studies of the performance of PA oximetry catheters have relied heavily on the use of regression analysis and correlation coefficients as a method of comparison. Clearly, each of these systems correlate with CO-oximetry and all are capable of reflecting Sv02. It has been suggested, however, that correlation coefficients constitute an inappropriate statistical tool for assessing agreement.

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Light of appropriate wavelength(s) is generated by light-emitting diodes and delivered to the PA by a fiberoptic bundle contained in the catheter. The light is backscattered off the hemoglobin in the red blood cells present in the circulation, and the reflected light is collected by a detecting fiberoptic bundle also contained in the PA catheter. Analysis of the reflected light and the application of appropriate algorithms permit the calculation of the relative proportions of oxyhemoglobin and deoxyhemoglobin.
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This measure of performance of the three catheter systems showed the greatest agreement among the three groups. The ESD for each group is also shown in Table 4. The 95 percent confidence limits based both on total variability and intrasubject variability are shown. The 95 percent confidence limits based on intrasubject variability are similar for the Oximetrix 3, SAT-2, and HEM0PR02 (±4.59, ±5.66, and ±6.56, respectively) while the 95 percent confidence limits based on total variability are more disparate (±6.03, ±6.86, and ±10.30, respectively).
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For example, patient 2 of the Oximetrix 3 group had a bias for the study period of +2, while the average bias for all patients in that group was —1.98. Similarly, the individual precision for some patients was considerably better than the precision for the group as a whole. When similarity of catheter performance to manufacturers’ specifications was tested (Eq 10 to 12) (±2 percent saturation, ± 1SD), none of the systems rejected the null hypothesis to prove similarity at ±2 percent relative to CO-oximetry.
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The 95 percent confidence limits are determined by the scatter along the y-axis, which is a function of the (in vivo — in vitro) values. Whether values are plotted against in vitro values or against (in vivo + in vitm/2) determines the position along the x-axis, and does not affect y-axis scatter of 95 percent confidence limits.
The data by time for the 24-h study period are shown in Figure 2. Bias and precision for each oximetry system relative to CO-oximetry are shown at each study period. Neither the Oximetrix 3 nor the SAT-2 systems demonstrated a significant change in bias (p = 0.882 and p = 0.15, respectively) over time. The HEM0PR02, however, did exhibit a statistically significant change in bias (p<0.001) during the 24-h study period.
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It has been suggested that a plot of the difference between methods against their means may be more informative and eliminates the statistical artifact caused by plotting the difference against either value separately. While this concept has been discussed in statistical publications, it is not widely recognized by medical users of statistics.
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Results
Patient demographics, catheter insertion site, and catheter tip location for each of the three groups are summarized in Table 1. The characteristics of each group were similar except for the relative proportion of men vs women in each group. Physiologic variables for each of the three groups during the course of the study are shown in Table 2. There were no significant differences among the groups for any of the variables recorded, Table 3 shows the mean, SD, and range of in vitro Sv02 and hemoglobin for each patient in each group. The group values for Sv02 are also shown in this table.
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A 95 percent confidence interval was determined for each catheters average bias using two methods. The first method used the SD, a, of all differences over all patients and the standard normal distribution (mean bias ±1.965 a). This method assumes that the measurement error is independent of the patient measured. The second method used the ESD as the variance estimate, thereby factoring out the intersubject variability. A Students t distribution with (10-1) degrees of freedom was used to construct this interval (mean bias ± 2.262 ESD). Trends over time were assessed using a univariate repeated measures of analysis of variance for the difference.
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Samples were processed immediately on an IL-282 CO-oximeter for total hemoglobin, oxyhemoglobin, deoxyhemoglobin, methemoglobin, and carboxy-hemoglobin determinations. No recalibrations were done during the study period in order to establish whether there were tendencies for in vivo SvO to drift over time relative to in vitro SvO,.
All patients enrolled in the study were considered to require hemodynamic monitoring for optimal clinical care. The physicians caring for each patient were given access to the data collected and the information was available to guide therapeutic interventions.
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